Refactor BlockingCell with modern concurrency utilities

"Modern" as in "from the 21st century".

Author: acogoluegnes

src/main/java/com/rabbitmq/utility/BlockingCell.java

@@ -16,39 +16,34 @@
 
 package com.rabbitmq.utility;
 
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
 
 /**
  * Simple one-shot IPC mechanism. Essentially a one-place buffer that cannot be emptied once filled.
  */
 public class BlockingCell<T> {
-    /** Indicator of not-yet-filledness */
-    private boolean _filled = false;
 
-    /** Will be null until a value is supplied, and possibly still then. */
-    private T _value;
+    private final AtomicReference<T> value = new AtomicReference<>();
+    private final CountDownLatch latch = new CountDownLatch(1);
 
     private static final long NANOS_IN_MILLI = 1000L * 1000L;
 
     private static final long INFINITY = -1;
 
-    /** Instantiate a new BlockingCell waiting for a value of the specified type. */
-    public BlockingCell() {
-        // no special handling required in default constructor
-    }
-
     /**
      * Wait for a value, and when one arrives, return it (without clearing it). If there's already a value present, there's no need to wait - the existing value
      * is returned.
      * @return the waited-for value
      *
      * @throws InterruptedException if this thread is interrupted
      */
-    public synchronized T get() throws InterruptedException {
-        while (!_filled) {
-            wait();
-        }
-        return _value;
+    public T get() throws InterruptedException {
+        this.latch.await();
+        return this.value.get();
     }
 
     /**
@@ -60,30 +55,27 @@ public synchronized T get() throws InterruptedException {
      * @return the waited-for value
      * @throws InterruptedException if this thread is interrupted
      */
-    public synchronized T get(long timeout) throws InterruptedException, TimeoutException {
+    public T get(long timeout) throws InterruptedException, TimeoutException {
         if (timeout == INFINITY) return get();
 
         if (timeout < 0) {
             throw new IllegalArgumentException("Timeout cannot be less than zero");
         }
 
-        long now = System.nanoTime() / NANOS_IN_MILLI;
-        long maxTime = now + timeout;
-        while (!_filled && (now = (System.nanoTime() / NANOS_IN_MILLI)) < maxTime) {
-            wait(maxTime - now);
-        }
+        boolean done = this.latch.await(10, MILLISECONDS);
 
-        if (!_filled)
+        if (!done) {
             throw new TimeoutException();
+        }
 
-        return _value;
+        return this.value.get();
     }
 
     /**
      * As get(), but catches and ignores InterruptedException, retrying until a value appears.
      * @return the waited-for value
      */
-    public synchronized T uninterruptibleGet() {
+    public T uninterruptibleGet() {
         boolean wasInterrupted = false;
         try {
             while (true) {
@@ -128,21 +120,19 @@ public synchronized T uninterruptibleGet(int timeout) throws TimeoutException {
                 Thread.currentThread().interrupt();
             }
         }
-
         throw new TimeoutException();
     }
 
     /**
      * Store a value in this BlockingCell, throwing {@link IllegalStateException} if the cell already has a value.
      * @param newValue the new value to store
      */
-    public synchronized void set(T newValue) {
-        if (_filled) {
+    public void set(T newValue) {
+        if (this.value.compareAndSet(null, newValue)) {
+            this.latch.countDown();
+        } else {
             throw new IllegalStateException("BlockingCell can only be set once");
         }
-        _value = newValue;
-        _filled = true;
-        notifyAll();
     }
 
     /**
@@ -151,10 +141,11 @@ public synchronized void set(T newValue) {
      * @param newValue the new value to store
      */
     public synchronized boolean setIfUnset(T newValue) {
-        if (_filled) {
+        if (this.value.compareAndSet(null, newValue)) {
+            this.latch.countDown();
+            return true;
+        } else {
             return false;
         }
-        set(newValue);
-        return true;
     }
 }

src/main/java/com/rabbitmq/utility/BlockingValueOrException.java

@@ -21,11 +21,11 @@ public class BlockingValueOrException<V, E extends Throwable & SensibleClone<E>>
     extends BlockingCell<ValueOrException<V, E>>
 {
     public void setValue(V v) {
-        super.set(ValueOrException.<V, E>makeValue(v));
+        super.set(ValueOrException.makeValue(v));
     }
 
     public void setException(E e) {
-        super.set(ValueOrException.<V, E>makeException(e));
+        super.set(ValueOrException.makeException(e));
     }
 
     public V uninterruptibleGetValue() throws E {